Among the halogenated olefins, tetrafluoropropenes are known to be useful in numerous applications. For example, U.S. patent application Ser. No. 10/694,273, which is assigned to the assignee of the present invention and incorporated herein by reference, discloses the use of CF3CH═CFH as a refrigerant with low global warming potential and also as a blowing agent for use in connection with the formation of various types of foams. In addition, CF3CH═CFH can also be functionalized to variety of compounds useful as intermediates for making industrial chemicals.
Applicants have come to recognize, however, that certain geometric isomers of halogenated olefins are more preferred than others in certain embodiments. For example, the trans-form of tetrafluoropropene is more preferred in certain refrigerant applications than the cis-form. Applicants have also come to recognize that many of the current processes for producing halogenated olefins, and in particular tetrafluorinated propene, produce a mixture of the geometric isomers. As a result, applicants have discovered a need for processes which are capable of converting one geometric isomer of a halogenated olefin to a more desirable geometric isomer.
U.S. Pat. No. 6,734,332—Slaugh et al is directed to a method for enriching the concentration in a product stream of one geometric isomer relative to another. More specifically, this patent introduces a stream comprising a mixture of geometric olefinic isomers (cis- and trans-forms) into an adduct reaction zone where it is contacted with a linear polyaromatic compound which tends to preferentially bind to the desirable form of the geometric isomer. The adduct thus formed is thereafter readily separated from the less desirable geometric form, and subsequently the adduct is disassociated and separated to form a stream rich in the more desirable geometric form. One failure of this process, however, is that it does not have the advantage of producing further quantities of the desirable geometric form. Rather, this process involves substantial processing steps, and the associated costs thereof, to simply separate the existing geometric forms from one another.
U.S. Pat. No. 5,672,803 describes a method for selectively producing a trans-geometric isomer of non-conjugated diolefins. The process involves reacting an isomerizable non-conjugated diolefin with a combination of other olefinic compounds under catalytic conditions to selectively form the trans-non-conjugated diolefin isomer. The patent discloses that the preferred catalyst is rhenium, with tungsten, molybdenum, cobalt, tantalum, niobium and chromium been disclosed also as potential catalysts for the reaction. One drawback of this method, however, is that it is relatively complicated insofar as it requires that several molecular species are fed to the reaction system. This is a drawback not only from the standpoint of increasing the complexity of the process scheme, but also from the standpoint of raw material costs and subsequent separation equipment. In addition, the process disclosed in the U.S. Pat. No. 5,672,803 appears to be limited to isomerization of non-conjugated diene olefins.
The effectiveness of transition metal chlorides as isomerization catalysts was examined by K. Endo, S. Okayama and T. Otsu in connection with the monomer-isomerization polymerization of cis-2-butene. Applicants are unaware of any disclosed process for the cis- to trans-isomerization of halogenated C2-C6 olefins, and the present invention resides, at least in part, from the recognition of a need for such a process and the development of effective and efficient processes for performing such isomerization, particularly the conversion of cis-1,3,3,3-tetrafluoropropene to trans-1,3,3,3-tetrafluoropropene.